[WIP] Tracking all the unsolved variables that was assigned ! type because of fallback

[blitzerr]

Addresses #66173

This is the first part. It will be followed by a commit that will separate out the live nodes and then only lint on the live nodes.

[rust-highfive]

r? @lcnr

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[blitzerr]

r? @nikomatsakis

[nikomatsakis]

Thanks @blitzerr . Marking as WIP for now since it doesn't (yet) address its stated goals.

[nikomatsakis]

OK so the only failure at this point is weird-exprs.rs...

warning: resulted from diverging fallback: ()
  --> $DIR/weird-exprs.rs:40:37
   |
LL |                           if (return) {
   |  _____________________________________^
LL | |                             return
LL | |                         } else {
   | |_________________________^

I guess that this happens because the result of if return { return } else { return } winds up being diverging fallback, and the if is live on entry (but not live on exit). I'm wondering if we can tweak the output to capture this sort of case...

[crlf0710]

This needs more discussions, so i'll convert this PR to draft to hide it from triaging list. Feel free to change it back when CI is green!

[bors]

☔ The latest upstream changes (presumably #74862) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

Discussed in the rust-lang/rust meeting:

• Conclusion was that the false warning from false-exprs.rs might be ok, but we should see if we can easily avoid it.
• We might be able to issue warnings only for expressions that are live on exit, not on entry (@blitzerr I'll ping you with some thoughts after meeting).
• Else we can rebase and do a crater run (but we have to change lint to deny by default).

[blitzerr]

After the latest rustc refactoring, the files have changed to:

• compiler/rustc_typeck/src/check/mod.rs
• compiler/rustc_typeck/src/check/writeback.rs

[bors]

☔ The latest upstream changes (presumably #76906) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

[bors]

☔ The latest upstream changes (presumably #77926) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author